personally i think and kind of sand mix will do fine. some have got fancy with their fillers but any cheap solution will do well. the good thing about the epoxy is it only shrinks about .001 and has about the same compressive properties as concrete. early 1900's they did beds for lathes with concrete and from what i read they did well. so if you have epoxy already that would be a cheap solution. 350 lbs is not very heavy at all so you will need some extra help at vibration dampening. your machine could cut as is possibly but i would not want to sit next to it very long will the frame sings to me

i ended up casting about 2100 lbs of E.G. into my frame and once i do my table top i will have a machine that weighs somewhere around 4250 to 4500 lbs. after 30 years of metal working i have no love for chatter or things vibrating obnoxiously